This invention relates to solid-state angular rate sensors and manufacturing methods therefor.
A conventional form of angular rate sensor employs a gyroscope to detect the rate of angular movement to which the sensor is subjected, utilising the precession effect of a rapidly spinning flywheel. Such angular rate sensors have been provided on vehicles of all kinds, and so including aircraft and boats, to assist with the guidance of the vehicle or to enhance the safety or performance of the vehicle. For example, the sensors may be used to control active suspension systems in automobiles or other passenger-carrying vehicles, the navigational control of aircraft and ships, in global positioning systems (GPS) to maintain tracking in the event that contact with satellites is lost, roll safety protection in vehicles and, increasingly, in model aircraft to assist with the remote control thereof.
Angular rate sensors provide an output from which the angular rate of motion of the sensor can be derived, or the angular displacement of the sensor, by integrating the output. A mechanical gyroscopic angular rate sensor has limited accuracy because of a number of defects inherent in the fundamental design thereof. In particular, a gyroscope is relatively large and heavy and power must continuously be supplied to cause rotation of the flywheel in view of the friction in the flywheel bearings and air resistance. Also, the gimbal assembly used to support the flywheel and permit the flywheel to turn in all directions introduces friction which reduces the accuracy of the sensor and if displacement is required, these errors accumulate over time. Further, a gyroscopic sensor is prone to drift, so reducing accuracy.
Over the last thirty years or so, there have been many proposals for solid-state devices able to detect angular rates of movement. These devices employ a resonating structure and the Coriolis forces generated by an applied angular rate affect the resonant mode of the structure. This can be detected and used to indicate the angular rate of movement of the sensor, and so to the displacement by integrating the rate signal.
In EP-0153189-A, there are described angular rate sensors in the form of both a three-dimensional cylinder and a two-dimensional disc, fabricated from a piezo-electric material, and which largely resolve the difficulties associated with the previously known solid-state devices. With both designs, the sensor is excited into the n=2 mode of vibration by a suitable drive signal applied to electrodes provided on the piezo-electric material and detecting the rate of turn by other appropriately positioned electrodes.
Angular rate sensors as described in said EP-0153189A can be made relatively small and lightweight as compared to gyroscopic devices, and need only a low drive current. Unfortunately though, these devices are expensive to manufacture and in particular must largely be trimmed by hand, in the final stages of assembly. Consequently, consistent performance is difficult to achieve.
In U.S. Pat. No. 4,655,081-A there is described a disc-shaped gyroscopic device, also fabricated from a piezoelectric material and having on one face an electrode pattern and on the other face a continuous earth electrode. The disc is significantly less stiff in the z direction as compared to the x and y (in-plane) directions and the absence of rigidity in the z direction leads to an overwhelming out-of-plane signal, which will swamp the in-plane n=2 mode.
EP-0823616-A describes a gyroscope formed from two pieces of ceramic material with an intermediate central electrode, with a patterned electrode structure on one outer surface of the device and a plane earth electrode on the other outer surface. This device is designed to operate solely in the flexural mode and is in effect a beam which bends in one plane when subjected to acceleration.